The only thing you should need to do is add the DPI option. Per the driver README:
Option "DPI" "string"

This option specifies the Dots Per Inch for the X screen; for example:

Option "DPI" "75 x 85"

will set the horizontal DPI to 75 and the vertical DPI to 85. By default,
the X driver will compute the DPI of the X screen from the EDID of any
connected display devices. See Appendix Y for details. Default: string is
NULL (disabled).